The London Journal was released in French as "Les papiers de Boswell: Amours à Londres 1762 - 1763" in 1952. It is probably available via the Abebooks used books search engine. A few other volumes of his journals are available as well, including "Boswell in Search of a Wife 1766-1769" translated as "Boswell veut se marier". Try searching Abebooks for "Les papiers de Boswell" and all available editions should come up.